Nov 20, 2017 Yikes! That’s even more harsh on my eyes! Don’t stare too long! What else can I change it to? I might be tempted to just plug in values until I get lucky, but there’s a better way.
Like many other languages, PowerShell has commands for controlling the flow of execution within your scripts. One of those statement is the switch statement and in PowerShell, it offers features that are not found in other languages. Today we will take a deep dive into working with the PowerShell switch.
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- Jan 12, 2018 This adds a lot of complexity and can make your switch hard to read. In most cases where you would use something like this it would be better to use if and elseif statements. I would consider using this if I already had a large switch in place and I needed 2 items to hit the same evaluation block.
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- Switch statement
- Arrays
- Parameters
- Advanced details
- Other patterns
One of the first statements that you will learn is the if statement. It lets you execute a script block if a statement is $true.
You can have much more complicated logic by using elseif and else statements. Here is an example where I have a numeric value for day of the week and I want to get the name as a string.
It turns out that this is a very common pattern and there are a lot of ways to deal with this. One of them is with a switch.
The switch statement allows you to provide a variable and a list of possible values. If the value matches the variable, then it’s scriptblock will be executed.
For this example, the value of $day matches one of the numeric values, then the correct name will be assigned to $result. We are only doing a variable assignment in this example, but any PowerShell can be executed in those script blocks.
Assign to a variable
We can write that last example in another way.
We are placing the value on the PowerShell pipeline and assigning it to the $result. You can do this same thing with the if and foreach statements.
Default
We can use the default keyword to identify the what should happen if there is no match.
Here we return the value Unknown in the default case.
Strings
I was matching numbers in those last examples, but you can also match strings.
I decided not to wrap the Component,Role and Location matches in quotes here to hilight that they are optional. The switch treats those as a string in most cases.
One of the cool features of the PowerShell switch is the way it handles arrays. If you give a switch an array, it will process each element in that collection.
If you have repeated items in your array, then they will be matched multiple times by the appropriate section.
PSItem
You can use the $PSItem or $_ to reference the current item that was processed. When we do a simple match, $PSItem will be the value that we are matching. I will be performing some advanced matches in the next section where this will be used.
A unique feature of the PowerShell switch is that it has a number of switch parameters that change how it performs.
-CaseSensitive
The matches are not case sensitive by default. If you need to be case sensitive then you can use -CaseSensitive. This can be used in combination with the other switch parameters.
-Wildcard
We can enable wildcard support with the -wildcard switch. This uses the same wildcard logic as the -like operator to do each match.
Here we are processing a message and then outputting it on different streams based on the contents.
-Regex
The switch statement supports regex matches just like it does wildcards.
I have more examples of using regex in another article I wrote: The many ways to use regex.
-File
A little known feature of the switch statement is that it can process a file with the -File parameter. You use -file with a path to a file instead of giving it a variable expression.
It works just like processing an array. In this example, I combine it with wildcard matching and make use of the $PSItem. This would process a log file and convert it to warning and error messages depending on the regex matches.
Now that you are aware of all these documented features, we can use them in the context of more advanced processing.
Expressions
The switch can be on an expression instead of a variable.
Whatever the expression evaluates to will be the value used for the match.
Multiple matches
You may have already picked up on this, but a switch can match to multiple conditions. This is especially true when using -wildcard or -regex matches. Be aware that you can add the same condition multiple times and all of them will trigger.
All three of these statements will fire. This shows that every condition is checked (in order). This holds true for processing arrays where each item will check each condition.
Continue
Normally, this is where I would introduce the break statement, but it is better that we learn how to use continue first. Just like with a foreach loop, continue will continue onto the next item in the collection or exit the switch if there are no more items. We can rewrite that last example with continue statements so that only one statement executes.
Instead of matching all three items, the first one is matched and the switch continues to the next value. Because there are no values left to process, the switch exits. This next example is showing how a wildcard could match multiple items.
Because a line in the input file could contain both the word Error and Warning, we only want the first one to execute and then continue processing the file.
Break
A break statement will exit the switch. This is the same behavior that continue will present for single values. The big difference is when processing an array. break will stop all processing in the switch and continue will move onto the next item.
In this case, if we hit any lines that start with Error then we will get an error and the switch will stop. This is what that break statement is doing for us. If we find Error inside the string and not just at the beginning, we will write it as a warning. We will do the same thing for Warning. It is possible that a line could have both the word Error and Warning, but we only need one to process. This is what the continue statement is doing for us.
Break labels
The switch statement supports break/continue labels just like foreach.
I personally don’t like the use of break labels but I wanted to point them out because they are confusing if you have never seen them before. When you have multiple switch or foreach statements that are nested, you may want to break out of more than the inner most item. You can place a label on a switch that can be the target of your break.
Enum
PowerShell 5.0 gave us enums and we can use them in a switch.
If you want to keep everything as strongly typed enums, then you can place them in parentheses.
The parentheses are needed here so that the switch does not treat the value [Context]::Location as a literal string.
ScriptBlock
We can use a scriptblock to perform the evaluation for a match if needed.
This adds a lot of complexity and can make your switch hard to read. In most cases where you would use something like this it would be better to use if and elseif statements. I would consider using this if I already had a large switch in place and I needed 2 items to hit the same evaluation block.
One thing that I think helps with legibility is to place the scriptblock in parentheses.
It still executes the same way and give a better visual break when quickly looking at it.
Regex $matches
We need to revisit regex to touch on something that is not immediately obvious. The use of regex populates the $matches variable. I do go into the use of $matches more when I talk about The many ways to use regex. Here is a quick sample to show it in action with named matches.
$null
You can match a $null value that does not have to be the default.
Same goes for an empty string.
Constant expression
Lee Dailey pointed out that we can use a constant $true expression to evaluate [bool] items. Imagine if we have a lot of boolean checks that need to happen.
This is a very clean way to evaluate and take action on the status of several boolean fields. The cool thing about this is that you can have one match flip the status of a value that has not been evaluated yet.
Setting $isEnabled to $true in this example will make sure the $isVisible is also set to $true. Then when the $isVisible gets evaluated, its scriptblock will be invoked. This is a bit counter-intuitive but is a very clever use of the mechanics.
$switch automatic variable
When the switch is processing its values, it creates an enumerator and calls it $switch. This is an automatic variable created by PowerShell and you have the option to manipulate it directly.
This was pointed out to me by /u/frmadsen
This will give you the results of:
By moving the enumerator forward, the next item will not get processed by the switch but you can access that value directly. I would call it madness.
Hashtables
One of my most popular posts is the one I did on everything you ever wanted to know about hashtables. One of the example use-cases for a hashtable is to be a lookup table. That is an alternate approach to a common pattern that a switch statement is often addressing.
If I am only using a switch as a lookup, I will quite often use a hashtable instead.
Enum
PowerShell 5.0 introduced the Enum and it is also an option in this case.
We could go all day looking at different ways to solve this problem. I just wanted to make sure you knew you had options.
The switch statement is simple on the surface but it offers some advanced features that most people don’t realize are available. Stringing those features together makes this into a really powerful feature when it is needed. I hope you learned something that you had not realized before.
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